A common cavity resonator is a quarter wave transverse-electromagnetic (TEM) coaxial resonator (“TEM resonator”). In the TEM resonator, the electric and magnetic fields lie in a transverse plane perpendicular to the conductors. The magnetic field is circular about the inner conductor. The electric field is axially symmetric about the inner conductor and extends from the inner conductor to the outer conductor. Current flows in the lengthwise direction along the surfaces of the conductors, in a direction perpendicular to both the electric and magnetic fields.
Another common cavity resonator is the waveguide cavity resonator. This type of resonator operates in a non-TEM mode, i.e., not transverse-electromagnetic. In a non-TEM mode resonator, both the electric and magnetic fields do not lie in a transverse plane perpendicular to the lengthwise conductors. In some modes, either the magnetic fields are transverse or the electric fields are transverse, but not both. A TEM mode resonator can also have waveguide modes at higher frequencies, but an empty waveguide cavity resonator cannot operate in the TEM mode. An empty waveguide guides the wave down its hollow inside from one end to another. By closing both ends of the waveguide, it resonates at frequencies determined by its inside dimensions. It has an extremely high Q and may be the highest Q cavity attainable, excluding superconductors. It is also the largest sized, at frequencies below about 1 GHz, its size generally prohibits its advantageous use.
Another cavity resonator is the evanescent mode cavity resonator. This type of resonator operates in a below cutoff waveguide cavity, i.e. below that frequency which an empty cavity would resonate. It is termed “evanescent” since the resonance is unsustainable in an empty cavity, and if excited in the empty cavity, the resonance would diminish rapidly. Above the cutoff frequency e.g., which depends on the dimensions, loading and other factors, the TEM coaxial cavity can also resonate in a waveguide mode. The evanescent mode is transitional between the TEM mode and the waveguide mode in a coaxial cavity resonator. Since it is intended to operate the cavity so that energy can be extracted from the cavity without loss of the energy into unwanted modes, prior art coaxial cavity have been designed with physical dimensions so that no waveguide modes can be excited, i.e., to operate strictly in the TEM mode.
A commonly used evanescent mode cavity is a metallic box that contains a metallic post, or dielectric resonator puck or post, or metallic post with a loading capacitor. Such posts and loading capacitors are used to lower the resonant frequency to below the frequency of the empty waveguide resonance and thereby reduce the size of the cavity. By enclosing a loading capacitor and metallic post in a below cutoff waveguide cavity, the resonant frequency is lowered, the Quality factor (Q) is raised higher than a quarterwave coaxial cavity, and the size is reduced. FIG. 8 shows an example of a conventional dielectric resonator filter, which is a ceramic puck resonating in a non-TEM mode within a below cutoff waveguide cavity.
Two common characteristics or specifications used to determine/specify the performance of a TEM resonator are the length of the resonator and the Quality factor (Q). The length is generally specified as a quarter, or three quarter wavelength. This reflects the fact that the length of the resonator post is one-fourth or three-fourths of the length of the wavelength at the resonant frequency. The resonator post is formed by electrically shorting or connecting one end of the line, and leaving the other end open or electrically disconnected. Using the above characteristics, a resonator can be designed to filter a particular frequency or range of frequencies.
The quality factor Q of the resonator describes the sharpness of the system's response to input signals. A general definition of the quality factor Q, that applies to acoustic, electrical, and mechanical systems, defines Q as equal to two times the product of the number π (pi) and the ratio of the maximum energy stored at resonance to the energy dissipated per cycle. In an electrical circuit, energy is stored in the electric or magnetic fields associated with reactive circuit components and electrical energy is lost (to heat) whenever current flows through a resistance.
Cavity filters can be used in various devices, including voltage controlled oscillators (VCO's), pagers, Global Positioning System (GPS) systems, TV/radio/cellular/PCS communications, magnetic-resonance imaging (MRI) systems, satellite transceivers, radars, radiometers, and the like in frequency ranges from 10 MHz to 10 GHz. A variety of military systems utilize these frequencies and many must be frequency-agile. Furthermore, the increasing needs of homeland security and the more than 20 million radio users in the United States are requiring that more communications equipment be added to already over crowded sites. In addition, the private radio systems utilized by commercial and public safety industries continue to face capacity restraints.
There is an increasing need for high Q cavity resonators of reduced size to be used as filters so the space saved can be used for additional equipment. In addition, cavity resonators with higher performance and lower cost are also required in order to work in more complex communication applications, such as narrowband digital frequency hopping radios. Such cavities need to be tunable to allow frequency adjustment, and temperature stable over their tunable range. Also, such cavities need to be easily connected and tuned in multiple sections, to give higher selectivity and performance and extend downward in frequency to the 100 MHz range, or lower.